Abstract To encode visual information about our surroundings, the visual system uses neurons that respond to features of the visual world?light, color, contrast, movement, orientation, speed, direction, and more. At the earliest stages (retina, thalamus) of the visual system, these features consist of relatively simple properties, such as spatial or temporal modulation of light, that are encoded as parallel channels. At later stages (primary visual cortex (V1), extrastriate regions), these simple properties are combined to represent more complex visual features, such as orientation, direction, and speed. How cortical circuits combine parallel inputs of simple visual features and transform them into new complex visual features is incompletely understood. One transformation that is highly relevant to many species is the identification of the speed of visual motion. The speed of an object by definition requires the processing of at least two inputs- its spatial location and its temporal displacement. These parameters can be studied quantitatively by using drifting sinusoidal gratings that vary systematically in spatial frequency (SF) and temporal frequency (TF). Each combination of SF (cycles/degree) and TF (cycles/second) corresponds to a particular speed (TF/SF = speed, degrees/second). In the visual system, there are neurons that respond best when objects are moving at a particular speed; in other words, they are ?speed tuned.? Some speed tuned neurons are thought to first emerge within V1. However, most are found in higher visual areas dedicated to motion processing. One hypothesis is that speed tuning emerges through a summation of offset SF and TF channels from V1 to higher visual areas. New transgenic and viral tools enabling cell type specificity as well as a recently improved understanding of the mouse visual system now make this a tractable hypothesis to test in mice. In mice, higher visual areas anterolateral (AL) and posteromedial (PM) are selectively tuned to different SFs, TFs, and speeds. AL is tuned to coarse features and fast speeds, while PM is tuned to fine features and slow speeds. Past studies have shown that AL and PM can inherit SF and TF tuning preferences from V1, but it is not known if they also inherit speed tuning or how speed tuning emerges. Preliminary data have also identified two cell populations in V1?s main thalamic input layer (layer 4) that are differentially tuned to SFs and TFs. This proposal hypothesizes that these V1 layer 4 populations represent parallel inputs that contribute differentially to the tuning of neurons in higher visual areas and the generation of speed tuning. The aims will first describe the speed tuning properties of V1, AL, and PM neurons in a laminar specific manner and evaluate the response properties of these layer 4 populations using awake in vivo extracellular electrophysiology and 2-photon calcium imaging. Finally, the hypothesis will be directly tested by selectively inactivating each layer 4 population and determining its effect on tuning in AL and PM. Together, these studies will have significant implications for understanding mechanisms of speed tuning in cortex and how complex sensory information is transformed across hierarchal cortical areas.